The optimization of two important classes of fed-batch fermentation processes with respect to the limiting substrate feed rate is considered:(i) growth of one biomass on one limiting substrate, and (ii) growth and product formation on one substrate. For these two types of bioprocesses, two different performance criteria are compared: yield and productivity. Heuristic feed rate profiles are derived based on (i) the optimal control solution according to the minimum principle, and (ii) the qualitative behavior of the specific rates for growth and possibly product formation. For biomass production processes, it is illustrated that the heuristic strategy coincides exactly with the optimal control solution, while its structure is independent of the performance index considered. For product formation processes, it is illustrated that the same heuristic approach can be used to optimize both yield and productivity in spite of the well known conflict between them.